In wireless systems, high frequency carrier signals are typically employed to carry relatively low frequency modulated packet data signals between a transmitter and a receiver. A mismatch in carrier frequencies can exist between the transmitter and receiver due to differences in local timing sources being employed by each of the transmitter and receiver. If uncorrected, this mismatch can result in poor receiver sensitivity and loss of packet data.
By way of example, the Wireless M-Bus (wM-Bus) standard (EN13757-4:2005 and 2012) specifies the radio frequency (RF) communication link between water, gas, heat, and electric meters and data collecting devices and is becoming widely accepted in Europe for smart metering or Advanced Metering Infrastructure (AMI) applications. Other applications for wM-Bus include alarm systems, flexible illumination installations, heating control, etc. Mode N of the wM-Bus specification has two use cases, 2.4 kbps and 4.8 kbps, and the standard outlines a frequency error tolerance requirement of +/−2kHz and +/−1.5kHz, respectively, for these data rates, i.e. the receiver/transmitter must be capable of communicating in spite of a 1.5/2 kHz difference in their respective reference frequencies.
At the same time, the wM-Bus mode N standard specifies a signal format comprising a relatively short preamble of 2 bytes and a 1 byte Synch Word (F6) to identify incoming data.
Within RF receiver modules, Automatic Frequency Correction (AFC) is carried out on a received signal to correct frequency mismatch between transmitter and receiver, typically during the preamble stage of a packet signal.
Clock and Data Recovery (CDR) is carried out to generate a receive clock and to ensure that received data bits are being sampled as near as possible to the centre of each bit, as this should ensure the best possible Signal to Noise Ratio (SNR) of the received bit.
For optimum performance, AFC and CDR should be settled before the data portion of a packet arrives. This can be challenging in systems using a short preamble, for example, the receiver sections of transceivers handing wM-Bus mode N signals.